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Last but not least, we come to the
fourth group of HTTP headers: entity headers. These headers provide
information about the resource carried in the body of an HTTP message,
called an entity in the HTTP standards. They serve the overall
purpose of conveying to the recipient of a message the information it
needs to properly process and display the entity, such as its type and
encoding method.

The most common type of entity is
a file or other set of information that has been requested by a client,
and for this reason, entity headers most often appear in HTTP responses.
However, they can also appear in HTTP requests, especially those using
the PUT and POST methods, which are the ones that transfer
data from a client to a server.

At least one entity header should
appear in any HTTP message that carries an entity. However, they may
also be present in certain responses that do not have an actual entity
in them. Most notably, a response to a HEAD request will contain
all the entity headers associated with the resource specified in the
request; these are the same headers that would have been included with
the entity, had the GET method been used instead of HEAD
on the same resource. Entity headers may also be present in certain
error responses, to provide information to help the client make a successful
follow-up request.

Note: Many of the entity headers have the same names as certain MIME headers, but they are often used in different ways. See the topic on HTTP Internet media types for a full discussion of the relationship between HTTP and MIME.

The following are the entity headers
defined in HTTP/1.1:

Allow

Lists all the methods
that are supported for a particular resource. This header may be provided
in a server response as a guide to the client regarding what methods
it may use on the resource in the future. The header must
be included when a server returns a 405 (Method Not Allowed)
response to a request containing an unsupported method.

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